It is desirable to optimize the magnetic circuit used in permanent magnet [PM] machines to obtain the highest power density and efficiency possible. Since PM machines typically have a relatively narrow high efficiency region on their fixed commutation (uncontrolled) torque vs. speed curve, many rotating machine technologists focus on increasing the motor size to power out ratio and the motor controller to enhance the overall performance of PM machines.
The focus on the controller to enhance the performance of PM machines is predicated on the belief that PM machines already operate at shear stress levels fairly close to their component materials' physical limits. This statement is misleading, however, since the true limiting factors are actually the permanent magnets and the geometry of the machine in which they are used, not the magnetically soft core materials as implied.
Wound field machines such as series wound, switched or variable reluctance machines can operate at shear stress levels at the material limits. Since these machines use field coils, rather than permanent magnets to produce the static magnetic field, the only limitation to the magnitude of the static field is the current carrying capacity of the copper wire. Such machines can reach the physical limits of the magnetically soft core material and produce high gap flux densities, but also result in increased I2R losses in the wound field coils and an increase in weight due to the field coils.
Permanent magnets are used in rotating machines to replace the field coils that produce static magnetic fields to provide three primary benefits:                1) A reduction in the size of the machine since the magnets are physically smaller than the coils they replace;        2) A reduction in the weight of the machine since the magnets are physically lighter than the coils they replace; and        3) The elimination of the I2R losses attributed to the field coils, thus reducing heat losses and therefore improving the overall machine efficiency.        
However, replacing a rotating machine's field coils with permanent magnets has the following trade-offs/limitations:                1) The energy product of a permanent magnet is fixed, thereby limiting the controllability of the static magnetic field;        2) State of the art permanent magnets cannot achieve the gap flux densities that can be achieved with wound field coils;        3) Permanent magnets do not make good structural components and can create bonding issues when placed on a machine's rotor;        4) Permanent magnets are more sensitive to temperature; and        5) The gap flux density is determined by the energy product of the permanent magnet and will always be less than Br or approximately 1.25 Tesla for neodymium magnets with a typical gap flux density of 0.8-˜1 Tesla with no power in the phase coils. The field intensity [H] of the phase coils will drive the gap density higher but cannot exceed ˜Br of the permanent magnets. If the flux in the air gap coupling a permanent magnet and a phase coil exceeds Br the amount greater than Br will be primarily uncontrolled fringing flux. The permanent magnet's domains between Br and Bmax require a greater amount of energy to bring them into temporary magnetic alignment.        
This is not meant to diminish the importance of the motor controller for improving performance but rather to state that mathematically, analytically and empirically it can be shown that the current and commonly accepted PM machine geometries cannot achieve the shear stress levels of wound field machines. The typical PM geometry is based on the concept of simply replacing a wound field coil with a PM without fully realizing or accepting the limitations and consequences of such a simplistic approach. Therefore, it would be desirable to provide an improved PM circuit motor.